|384 HETEROCHRONY AND PAEDOMORPHOSIS
senting this discussion, I have no intention of falling into the Bolkian trap of all or nothing; I merely want to argue that most of the classic "exceptions" to human paedomorphosis are really consequences of retarded development, the central phenomenon of our heterochronic evolution.
A hidden assumption of much writing on human paedomorphosis is that the great apes are appropriate surrogates for human ancestors. Human traits are judged as paedomorphic if they correspond to juvenile states of chimps and gorillas. The use of pongids as prototypes has inspired two very different, almost contradictory, objections: first, that they are inappropriate as prototypes; and second, that when they are employed as prototypes, they confute the hypothesis. I shall uphold the first objection, use it to deny the second, and then argue that the best prototype among living forms is the human fetus or juvenile itself.
We did not evolve from any primate bearing much resemblance to the adult forms of chimps and gorillas. The great apes, in their ontogeny, develop many peculiar features confined uniquely to them and having little to do with simian phases of human ancestry. If we are paedomorphic with respect to these specializations, this fact is irrelevant to events in our own phylogeny.
Many of the "simian" features that we have supposedly "avoided" by paedomorphosis are actually the consequences of a specialized adaptation in pongids: strong alveolar prognathism, with its correlates, including brow ridges and the sagittal crest (Naef, 1926a; Bolk, 1926c, p. 31; Scott, 1963). "This alveolar growth," Scott writes, "is of course related to the massive development of the dentition and appears to be a late specialization among the anthropoid apes" (1963, p. 131). Scott argues that the correlates of alveolar growth are necessary adaptations to support a massive dentition in animals that had already undergone a paedomorphic reduction of the face:
Massive alveolar processes are associated with large cheek teeth, prominent canines, powerful muscles of mastication, a well-developed simian shelf or mandibular torus, and well-developed brow ridges especially in animals with low retreating foreheads. Other features of the facial skeleton such as the nasal, lacrimal and ethmoid bones and turbinate processes are not involved in the development of this skeletal masticatory mask. The superimposed masticatory skeleton which reaches its greatest development in the male gorilla is a functional necessity absent in animals such as the dog and the pig in which the strong tubular snout, extending back between the orbital cavities, provides an adequate skeletal anvil to resist the moving mandible. (1963, p. 132)